Combined capacitive and inductive tuning unit



April 9, 1946. E O'BRIEN 2,398,112

' COMBINED CAPACITIVE AND INDUCTIVE TUNING UNIT Filed Aug. 4, 1944 lZa T "1217 12d WAVE-SIGNAL TRHNSLH TING CIRCUIT INVENTOR.

Patented Apr. 9, 1946 UNITED STATES PATENT OFFICE COMBINED CAPACITIVE AND INDUCTIVE TUNING UNIT Application August 4, 1944, Serial No. 548,096

3 Claims.

This invention relates to combined capacitive and inductive tuning units and, while it is of general application, it is particularly suitable for use in tuning a wave-signal apparatus over a wide range of radio frequencies.

Heretofore tunable radio-frequency circuits have generally comprised inductance and capacitance elements, only one of which has been adjustable to tune the circuit over a desired range of frequencies. 7 such a circuit depends upon the ratio of the maximum value to the minimum value of the adjustable tunable element. For practicable commercial structures. it is generally found that the limit on the ratio of maximum value to minimum value is of the order of 9 or 10 to 1, due to the unavoidable minimum value of the inductance or the capacitance of the element and its associated circuit elements. This corresponds to a frequency range of approximately 3 to 1. frequency range has been required, resort has been had to switching the values of one or the other of thetuning elements or to adjusting both 01' the tuning elements simultaneously in the same sense. The first-named expedient is inherently complicated and has the well known disadvantages attendant upon the use of movable contacts and at the same time gives a discontinuous tuning control. On the other hand, the apparatus heretofore proposed for use as the second-named expedient has generally been complicated mechanically as well as bulky and costly.

It is an object of the invention, therefore. to provide a new and improved combined capacitive and inductive tuning unit suitable for tuning a wave-signal apparatus continuously over a Wide band of radio frequencies which is simple, inexpensive and compact in construction.

In accordance with the invention, a combined capacitive and inductive tuning unit comprises a supporting frame, a rotatable tuning condenser mounted thereon and having an actuating shaft and a camming element mounted on said shaft and having a helical cam surface. The tuning .unit also comprises an inductor including an elongated winding element and an elongated core element, one element being supported from the frame and the other element being axially adjustable relative thereto for adjusting the induc tance value thereof. The winding and core ele ments are parallel to the axis of the shaft and there is provided a driving connection between the camming element and the adjustable element, the driving connection being such that the values The maximum tuning range of When a wider of the condenser and inductor are simultaneously adjustable in the same sense.

Referring now to the drawing, Fig. 1 is a side elevation, partly broken away, of a combined capacitive and inductive tuning unit constructed in accordance with the invention; Fig. 2 is a top detail view of a portion of the apparatus of Fig. 1, while Fig. 3 is a simplified schematic circuit diagram illustrating an application of the combined capacitive and inductive tuning unit of the invention.

Referring now to Figs. 1 and 2 of the drawing, there is illustrated a combined capacitive and inductive tuning unit comprising a. supporting boxlike frame Hi, the front wall of which is broken away for clarity of illustration. Mounted on the frame I0 is a rotatable adjustable tuning condenser H which may be of conventional structure including a supporting frame Ila riveted or otherwise secured to the frame I!) and provided with an actuating shaft l lb to which is connected any suitable operating means such as a knob Ho. The tuning unit also comprises a camming element having a driving connection with the condenser shaft l lb and of a configuration for converting the rotary motion of the adjust-able condenser into translatory motion and vice versa. For example, the camming element may comprise a cylindrical member I! mounted on the shaft Hb by means of cup-like end pieces lZa, lZb, the piece l2a being provided with a hub lZc for looking to the shaft llb by suitable means such as a set screw I211. The member l2 has a helical cammlng element either in the form of a helical rib or, as illustrated, a helical cam slot lZe formed in the surface thereof.

The tuning unit also includes an inductor consisting of at least two elements, one element being supported from the frame land the other element being axially adjustable relative thereto for adjusting the inductance value thereof. More specifically, the inductor comprises an elongated winding element l3 wound on a suitable form l3a of insulation material and terminating in an enlarged head or base |3b suitably supported from the frame I 0 by rivets, screws or the like, with its axis parallel to the axis of the condenser shaft Mb. The other element of the inductor comprises an elongated magnetic core element l4, preferably in the form of a plug of powdered iron compressed in a binder. The axes of the winding l3 and core I4 are both parallel to the axis of the condenser shaft llb, as illustrated. In order to adjust the core I4 axially relative to the winding l3, there is provided an elongated guide means supported from the frame. The guide means may comprise a pair of elongated slots in opposed side plates of frame II) or, as illustrated, a pair of guide rods I disposed parallel to the axis of the winding I3 and secured in the frame by riveting or upsetting the ends thereof in the opposite ends of the frame IE1. There is also provided a carriage slidably supported on the guide rods and comprising a crosspiece IS with an angleshaped cross section to which are secured a pair of sleeves l1 mounted on the guide rods I5. The core member 14 is provided with a threaded X- tension Md molded or otherwise secured into the end thereof and this threaded extension is engaged. by a pair of spring clips Isa riveted or otherwise secured to the portion of the frame I6 normal to the guide rods I5. The core I4 may be axially adjusted relative to the cam slot I 26 by the threaded. extension Ida for alignment purposes. As thus supported on the carriage Iii, Il the magnetic core element I4 is axially adjustable relative to and within the winding I3 for adjusting its inductance value in a manner well understood in the art. It will be apparent, how ever, that the core member I4 ma be stationary and the winding I3 axially movable relative thereto, if desired.

The tuning unit also includes a driving conection between the camming means or member i2 and the adjustable core I4 of the inductor element. Specifically there is provided a pin I3 projecting upwardly from the flange of the carriage i6 and secured thereto b riveting, upsetting. or the like, this pin extending through a slot ice. in the upper wall of the frame Ill and engaging the camming means as by extending into the cam slot i2 3. In order substantially to eliminate any backlash, the pin I8 is split and slightly spread and provided with small spherical projections 53a for resiliently engaging the walls of the cam slot I22, thereb reducing friction and increasing the accurac of adjustment.

The driving connection between the condenser shaft lb and the camming element, in this case the mounting of the cam cylinder I2 upon the condenser shaft and the configuration of the eamming element, that is, the pitch of the cam slot are such that the values of the capacitance oi the condenser l i and the inductance of the winding I3 are adjustable in the same sense by adjustment of the knob He. The cam slot I26, while referred to a helical slot. need not be the ideal mathematical helix but the term is used only to indicate the general type of curve involved.

The configuration of the cam slot 12c and the shape of the plates of condenser I I may be selectod or designed in accordance with the particular variation of inductance and capacitance desired with adjustment of the tuning unit by the knob i to. For example, the cam slot '26 and the plates of the condenser ii may be designed to obtain either straight-line frequency variation or straight-line wave length variation with adjustment of the knob H c. Alternatively, there may be obtained a constant frequency difference between the circuit comprising the tuning unit and another tunable circuit unicontrolled by the same tuning knob Hc as. for example, is desirable in tracking the oscillator circuit of a superhetelodyne receiver with the tunable radio-frequency circuits. Furthermore, the cam slot I 26 may be so proportioned in relation to the shape of the plates of the condenser l I as to impart to the circuit comprising the tuning unit a constant impedance, that is, a constant L/C ratio.

An insulated terminal llcZ of condenser II and end terminals I3c and an intermediate terminal It'd of the winding !3 may be brought out to a plurality of terminals I9 supported on a terminal plate 20 of insulation material suitabl secured to the frame I9. However, it will be understood that, in case other components of the apparatus in which the tuning unit is embodied are to be supported in or from the frame In, certain of the terminals of the condenser II and the winding I3 may be appropriatel arranged for connection to their associated elements.

In Fi 3, there is represented schematically a simplified circuit diagram illustrating an application of the tuning unit of the invention. In this case, the elements of the tuning unit are connected to form a tunable circuit 2| for connection to the antenna 22 of a wave-signal translating circuit 23 which ma be either a transmitter or a receiver. Simultaneous adjustment of the inductance of the inductor I3 and the capacitance of condenser H is effective to adjust the tuning of the circuit 2i over a wide band of radio frequencies. In a particular apparatus in which he tuning unit of the invention has been embodied, the tuning range was from 14 to 50 megacycles.

While there has been described what is at present considered to be the preferred embodiment of the invention, it will be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit or scope of the invention.

What is claimed as new is:

l. A combined capacitive and inductive tuning unit comprising, a supporting frame, a rotatable tuning condenser mounted thereon and having an actuating shaft, a camming element mounted on said shaft and having a helical cam surface, an inductor including an elongated winding element and an elongated magnetic core element, one element being supported from said frame and the other element being axially adjustable relative thereto for adjusting the inductance value thereof. said winding and core elements being parallel to the axis of said shaft, and a driving connection between said camming element and said adjustable element, said driving connections being such that the values of said condenser and said inductor are simultaneously adjustable in the same sense.

2. A combined capacitive and inductive tuning unit comprising, a supporting frame, a rotatable tuning condenser mounted thereon and having an actuating shaft, a camming element mounted on said shaft and having a helical cam surface, an inductor including an elongated Winding element supported from said frame, elongated guide means supported from said frame parallel to the axis of said winding element, an elongated magnetic core element slidably supported by said guide means and axially adjustable relative to said winding for adjusting the inductance value thereof, said winding and core elements being parallel to the axis of said shaft, and a driving connection between said camming element and said adjustable element, said driving connections being such that the values of said condenser and said inductor are simultaneously adjustable in the same sense.

3. A combined capacitive and inductive tuning unit comprising, a supporting frame, a rotatable tuning condenser mounted thereon and having an actuating shaft, a cylindrical member mounted on said shaft and having a helical cam slot formed therein, an inductor including a winding element supported from said frame with its axis parallel to the axis of said. condenser shaft, 2. pair of elongated guide rods supported from said frame parallel to the axis of said winding element, a carriage slidably supported on said guide rods, a magnetic core element supported on said carriage and axially adjustable relative to and Within said winding for adjusting the inductance value thereof, and a pin extending from said carriage and engaging said cam slot, the configuration of said cam slot being such that the values of said condenser and said inductor are simultaneously adjustable in the same sense.

EDGAR O'BRIEN. 

